System and method for sealing a tank opening

ABSTRACT

System for closing off an opening in a plastic tank, formed from a sheet of multilayer plastic that includes a barrier layer, the said sheet being welded around the perimeter of the opening and provided with a peripheral region that is slimmed down so as to effectively restrict the permeation of gases and liquids into the layer located between the tank and the barrier layer.  
     Method of closing of a tank by welding such a plate, comprising the welding of the slimmed-down peripheral part by the technique of laser or infrared radiation welding.  
     Use of the method to close off a fuel tank for a motor vehicle.

[0001] The present invention relates to a system for closing off anopening in a tank.

[0002] Liquid and gas tanks in use in industry or on board vehicles ofvarious kinds must in general satisfy sealing and permeability standardsrelating to the type of use for which they are designed and theenvironmental requirements that they must meet. At the present time,both in Europe and in the world, the requirements regarding thelimitation of emissions of pollutants into the atmosphere and into theenvironment in general are being considerably tightened. The design ofliquid and gas tanks is consequently undergoing rapid developmenttowards using techniques capable of better guaranteeing sealing andsafety under a variety of operating conditions. Moreover, it is alsoendeavoured to reduce as far as possible any losses arising from thevarious accessories and pipes associated with the tanks. One meanssometimes used has been to incorporate certain accessories and pipeswithin the tanks, thus eliminating their interfaces with the externalatmosphere.

[0003] It is known, from Patent Application WO 01/21428 by SOLVAY, toclose off an opening in a multilayer fuel tank by means of a multilayerplastic sheet welded to the wall of the tank so as to cover the opening,the layer of plastic of the sheet welded to the tank having acomposition compatible with the surface layer of the tank.

[0004] In the tank described in that patent application, there remains,however, a fuel permeation channel located between the barrier layer,with which the inside of the multilayer structure of the closure sheetsis generally provided, and the wall of the tank, the channel being madeof a plastic such as HDPE, the impermeability to fuel of which is notoptimal.

[0005] The object of the present invention is to provide a system forclosing off a tank which further reduces the losses of liquid and gascompared with the known closure systems and makes it possible to achieveloss levels low enough to meet the new environmental standards that areprogrammed to come into force in the near future both in Europe and theUnited States.

[0006] For this purpose, the invention relates to a system for closingoff an opening in a;plastic tank, formed from a plastic sheet welded tothe wall of the tank around the perimeter of the opening, the sheetbeing formed from a multilayer structure that includes a barrier layer,the outer layer of which structure, welded to the tank, has acomposition compatible with that of the plastic forming the outersurface of the tank, in which a peripheral region of the sheet, in theweld region, has been slimmed down so as to adapt its geometry accordingto the following relationship:

l/e≧5

[0007] in which l represents the width of the slimmed-down region and ethe thickness of the layer in contact with the wall of the tank, bothexpressed in the same length units.

[0008] Preferably, the l/e ratio is at least 8.

[0009] The term “tank” is understood to mean a closed chamber, ofvarious shapes, generally sealed with respect to the outside, whichmay-be equipped with various accessories internal to the chamber orpassing through its wall. The tank may be a gas tank, a liquid tank or atank designed to contain both a gas and a liquid. Preferably, it is aliquid tank or a liquid tank containing also a gaseous phase.

[0010] Certain tanks have one or more openings, that is to say cut-outsof circular or other shape, made through the wall of the tank forvarious purposes, for example so as to be able to introduce one-or moreaccessories into the tank that cannot be incorporated therein during itsmanufacture.

[0011] Before being able to use the tank for its normal application ofstoring and confining liquid and gas, it is therefore necessary todesign and fit a closure system capable of sealing the tank ashermetically as possible after the accessory has been introduced.

[0012] According to the invention, the closure system is designed forplastic tanks. The term “plastic” is understood to mean any syntheticpolymeric material, being thermoplastic or thermosetting in the solidstate under ambient conditions, and also blends of at least two of thesematerials. The polymers intended comprise both homopolymers andcopolymers (especially binary or ternary copolymers). Examples of suchcopolymers are, non-limitingly, random copolymers, linear blockcopolymers, other block copolymers, and graft copolymers. Thermoplasticpolymers, including thermoplastic elastomers, are preferred.

[0013] The plastic tanks according to the invention are preferably inthe form of tanks of multilayer structure. Tanks comprising at least onelayer of high-density polyethylene (HDPE) are particularly preferred.Particularly preferred tanks are those in which the structurefurthermore includes at least one internal layer made of a barriermaterial, that is to say a material, generally of polymeric nature,which has a very high impermeability to certain liquids and gases.

[0014] For example, it is possible to use known barrier compositionssuch as those used for making fuel tanks impermeable. Examples of suchbarrier compositions are, non-limitingly, resins based on polyamides orcopolyamides, ethylene/vinyl alcohol random copolymers (EVOH), or eventhermotropic liquid-crystal polymers (LCP) such as copolyesters ofp-hydroxybenzoic acid with either 6-hydroxy-2-naphthoic acid orterephthalic acid and with 4,4′-biphenol (for example, the copolyesterssold under the brand names VECTRA® and XYDAR®). The term “sheet” isunderstood to mean any solid element whose thickness is small comparedwith its area and whose rigidity is close to that of the walls of thetank. According to the invention, the material constituting the sheetcomprises a plastic. Preferably, the sheet forming part of the closuresystem according to the invention is produced exclusively from plastic.The sheet may come in various shapes; usually the shape of the sheetdepends on the various types of tank and on the position of the openingin the surface of these tanks that has to be closed off. The sheet oftenhas a plane or slightly curved surface, in particular in the case ofsmall openings such as, for example, circular openings with a diameternot exceeding 80 mm.

[0015] According to the invention, the sheet is welded to the wall ofthe tank around the perimeter of the opening. By this it should beunderstood that the sheet is fastened to this wall by bringing themolecules of a portion of the surface of one of the two main faces ofthe sheet adjoining the ends of the latter and forming a junctionsurface in the form of a closed ring, of circular or any other shape,and to a greater or lesser extent faithfully matching the outerperimeter of this face of the sheet, into contact and partialinterpenetration with the molecules of a similar surface of the tank,located around the border of the opening, over its entire perimeter.

[0016] According to the invention, the sheet is formed from a multilayerstructure, the outer layer of which, welded to the tank, has acomposition compatible with that of the plastic forming the outersurface of the tank.

[0017] The term “compatibility” is understood here to mean chemical andphysical compatibility of the constituents of the respectivecompositions of the layer of the sheet welded to the tank and of theouter wall of the latter in the region of the opening. Goodcompatibility prevents the phenomenon of segregation of certainconstituents of the respective compositions of the welded parts, andlikewise any chemical reaction and physical phenomenon prejudicial tothe properties of the tank whose opening is thus closed off. In general,good compatibility ensures long-term adhesion of the sheet to the tank.

[0018] One particular preferred system according to the invention isthat in which the tank includes, like the sheet, a barrier layerimpermeable to liquids and gases. The barrier layers of the sheet and ofthe tank may be identical or different. In general, it is preferred forthese barrier layers to be identical, particularly when they compriseidentical barrier compositions.

[0019] These barrier compositions may be incorporated into one of thelayers of the sheet or a special additional layer essentially consistingof a barrier composition may be inserted into the structure. Preferably,an additional layer essentially consisting of a barrier composition isinserted into the structure.

[0020] When the tank has a multilayer structure, the multilayerstructure of the sheet may be different from that of the walls of thetank itself, but may also be identical thereto. Closure systems in whichthe structure of the sheet is identical to that of the walls of the tankare often preferred. Most particularly preferred are systems in which,within identical structures, the barrier compositions themselves areidentical and constitute identical polymeric layers.

[0021] As a variant, the multilayer structure of the sheet may alsocomprise two different barrier layers. A preferred example according tothis variant is that of sheets having two complete structures identicalto that of the walls of the tank, stacked one on top of the other andresulting from the compression of two structures identical to the wallsof the tank.

[0022] Another variant consists in producing, near the edge of thesheet, an embossment in which the thicknesses of the layers are reduced,in particular those of the layers other than the barrier layer. Thebenefit is in this case to further limit the risk of loss bytransmission at the point where the sheet is welded to the tank.

[0023] The expression “outer layer of the sheet” is understood to mean alayer one of whose faces is adjacent to another layer of the sheet andwhose other face is in contact with the environment external to thissheet.

[0024] The system according to the invention is very suitable forclosing off a simple opening in a tank. It is also suitable for closingoff an opening above an accessory located in the tank.

[0025] According to one particularly preferred embodiment of the closuresystem according to the invention, the sheet is fastened to at least oneaccessory located on the inside of the tank. The term “accessory” isunderstood to mean in general any member via which liquid or gas passes,or one in contact with liquid or gas, and which fulfils a particularfunction specific to the device of which the tank forms part, includinga function of transporting liquid and/or gas between two other members.

[0026] Examples of such accessories comprise, not limitingly, thefollowing:

[0027] a container that contains any chemical or physical composition,especially a vapour absorption canister;

[0028] a liquid or gas gauge;

[0029] a electrical connection terminating in a liquid or gas gauge;

[0030] a liquid or gas pump;

[0031] a safety valve for controlled closure of the tank under certainspecial conditions;

[0032] a drainable container for receiving liquid;

[0033] an electrical connection for supplying the motor of a liquid orgas pump;

[0034] a liquid pipe terminating in a supply device for any device,especially an engine;

[0035] a liquid/vapour separation device.

[0036] Any combination of at least two accessories may be used,optionally in the presence of several examples of the same accessory.

[0037] According to this embodiment, the accessory is fastened to thesheet, that is to say it is linked to the sheet by a mechanical means.The accessory is fixed to the sheet by any mechanical means compatiblewith the type of tank, the nature of the liquid and the gas that arestored therein and the normal usage to which the tank is put. The sheetmay provide a means of gripping and transporting the accessory. Forexample, it is possible to use a sheet to which the accessory is fixedby a mechanical fixing device. Such devices are, for example, screws,bolts, nuts, clip systems, etc. It is also possible to use a sheet andan accessory that have been fastened together right from theirmanufacture, for example by moulding. It is also possible to make use ofa fastening means with a more permanent character, such as adhesivebonding or welding. In general, it is preferred for the accessories tobe fastened to the sheet by welding.

[0038] The accessory fastened to the sheet is present with respect tothe latter on that face of the sheet which is on the inside of the tankwhen the closure has been made.

[0039] Accessories of various types fall within this embodiment. Asexamples, mention may be made of ROV (Roll-Over Valve) valves, i.e.valves that will automatically close in the event of the tank turningupside down, ORVR (On-board Refuelling Vapour Recovery) valves, i.e.devices for vapour recovery when refuelling the tank, and OBD (On-BoardDiagnostic) devices, and liquid/vapour separators.

[0040] Some of these accessories may include at least one pipe passingthrough the sheet via an orifice drilled in the latter. It is oftenadvantageous in this case to provide the outlet orifice of the pipe witha seal.

[0041] The sheet may also have been formed so as to be tailored as faras possible to the surroundings of the opening to be closed off and tothe shape and size of the accessory to which it is fastened.

[0042] A sheet fastened to a liquid-vapour separator that includes avapour discharge pipe passing through the sheet via an orifice has givengood results.

[0043] One particular closure system consists in using a tank having awall that bears fixing lugs near the opening. These allow the wall to besupported during the welding operation, should this wall undergoflexible deformation.

[0044] The invention also relates to a method of closing off an openingin a plastic tank by means of a plastic sheet of multilayer structure,the outer layer of which, welded to the wall of the tank around theperimeter of the opening, has a composition compatible with that of theplastic forming the outer surface of the tank, in which a peripheralregion of the sheet is slimmed down, before welding, so as to adapt itsgeometry according to the following relationship:

l/e≧5

[0045] in which l represents the width of the slimmed-down region and eits thickness, both expressed in the same length units.

[0046] In this method, the specific terms and expressions that arecommon, such as tank, opening, closure, plastic, multilayer structure,sheet, welding, around the perimeter of the opening, compatiblecomposition, have exactly the same meanings as those mentioned above inthe case of the closure system.

[0047] The sheet used in this method may have come from various sources.For example, it may have been manufactured independently of the tankitself, possibly at a different time, by means of a specific processthat is not necessarily identical to that used to manufacture the tank.For example, the sheet may be obtained by coextrusion, optionallyfollowed by compression moulding. It may also have been obtained bymoulding using one of the many known techniques. Such techniques are,for example, blow moulding, compression moulding, injection moulding andthermoforming.

[0048] When the sheet is manufactured independently of the tank, anopening is made in the tank with dimensions that are slightly smallerthan those of the sheet, and then the opening is closed off by weldingthe sheet.

[0049] As a variant, the sheet may have been manufactured at the sametime as the tank and cut from the plastic of multilayer structure usedto manufacture the tank. This procedure is particularly advantageousfrom the standpoint of synchronising the production of tanks fitted withtheir accessories. It is possible, for example, to cut out the sheetdirectly from a region of the wall of the tank, after the latter or partof it has been produced. This variant assumes that the innermost outerlayer of the multilayer structure of the tank is compatible with itsoutermost layer.

[0050] Another variant is to cut out the sheet from tank manufacturingscrap, provided that this has the appropriate multilayer structure.

[0051] It is also possible to cut out the sheet from an ancillary partof the tank specially designed for this purpose and producedsimultaneously with it.

[0052] Finally, it is also possible to manufacture sheets comprising twocomplete structures identical to the structure of the walls of the tank,by pressing together two structures identical to the walls of the tank.In this case, it is possible, for example, to compress a particular partof the parison used for producing the tank. To do this, it is mostparticularly advantageous for the two identical structures of the sheetto be compressed using the same production mould as for the tank.

[0053] Any suitable technique may be used to produce the slimmed-downperipheral region of the sheet. Techniques that are well suited areforming techniques such as thermoforming and extrusion-compressionmoulding.

[0054] The operation of welding the sheet to the tank may be carried outby any technique generally known per se. For example, it is possible touse the hot-plate welding technique or the laser welding technique. Thehot-plate welding technique is preferred. The technique of welding theplate to the tank may be different from that of the slimmed-downperipheral region of this sheet.

[0055] According to the invention, the slimmed-down part of the sheet iswelded by a laser radiation or infrared radiation welding technique.

[0056] According to a preferred way of implementing the method accordingto the invention, a composition opaque to the laser radiation or to theinfrared radiation is incorporated into the plastic layer of theslimmed-down region in contact with the outer surface of the tank.

[0057] Advantageously, this method of implementation produces tanks inwhich the welding by laser or infrared radiation of the slimmed-downperipheral region of the sheet is of better quality. The expression“composition opaque to laser or infrared radiation” is understood tomean a composition that absorbs and/or reflects a substantial proportionof the laser or infrared radiation energy. An example of such acomposition that has given good results is a carbon black filler. Thiscomposition is advantageously distributed uniformly throughout thepolymeric material of the layer directly in contact with the wall of thetank.

[0058] According to one particular version of the method according tothe invention, at least one accessory is fastened to the sheet, on theinside of the tank. Any fastening method compatible with the tank, thenature of the liquid and the gas that it contains, and with itsoperating conditions may in general be suitable.

[0059] However, it is preferred to use a welding process to carry outthis fastening operation.

[0060] Here again, the terms and expressions “fastening” and “inside ofthe tank” have the same meaning as already explained above in the caseof the closure system.

[0061] In this particular way of implementing the method according tothe invention, a first variant consists in welding at least oneaccessory to the sheet before the opening in the tank is closed off bywelding the sheet fastened to the accessory around the perimeter of thisopening.

[0062] In this way, the sheet acts as support for the accessory andallows it to be easily introduced into the tank.

[0063] A second variant consists, in contrast, in fastening, in a firststep, at least one accessory to the internal wall of the tank that isadjacent to the opening and, in a subsequent second step, in closing offthe tank by simultaneously welding the sheet to the accessory and to theperimeter of the opening.

[0064] In one or other of the methods of implementation employed, oneproblem may arise when carrying out the operation of welding the sheetto the wall of the tank when this wall undergoes, for any reason,flexible deformation near the opening, for example owing to the effectof its own weight, as a result of forces exerted perpendicular to thesurface of the tank during welding. A similar situation may in factarise when the tank comes directly from manufacture and when thetemperature of its walls is still relatively high. Another reason couldbe that the diameter of the opening is relatively large and that theplastic near the edge of the opening is no longer supported aseffectively as in the case of small openings.

[0065] The invention also relates to a closure method that remediesthese problems of flexible deformation of the tank wall, by supportingthe tank wall in the region of the opening during the operation ofwelding the sheet by means of lugs moulded to this wall.

[0066] The lugs are moulded to the outer wall of the tank near theperimeter of the opening. The wall of the tank is supported at thispoint by means of clamps that grip the lugs.

[0067] Lugs similar to those described above may also be used when thesheet is being cut out from the wall of the tank, so as to prevent thiswall from deforming.

[0068] The lugs may also be used, before the welding operation, tocorrect a lack of flatness that may occur on the surface of the tank.

[0069] Finally, they may also help to bring the shape of the outersurface of the tank into perfect correspondence with that of the sheet.

[0070] The invention also relates to the use of the method describedabove to close off an opening in a fuel tank, particularly for a motorvehicle.

[0071] The term “motor vehicle” is understood to mean vehicles poweredby an internal combustion engine such as lorries, cars and motorcycles.

[0072] The figure that follows is given for the purpose of illustratingthe invention, without wishing to restrict its scope.

[0073] The figure shows a tank (11), the wall of which is formed from amultilayer structure comprising an EVOH barrier layer surrounded by twoHDPE layers. The barrier layer is fastened to the HDPE layers via a thininterlayer of 80 μm of an adhesive made of a maleic-anhydride-graftedpolyethylene of the ADMER® L2100 trade mark from Mitsui. The multilayerstructure of the wall of the tank (11) has an opening into which asiphon (2) has been snap-fastened onto the rim (5) of the wall (4) ofthe tank (11). A venting ROV valve (3) having a float (6), and providedwith a polyacetal skirt (3), was fixed to the siphon (2), again bysnap-fastening.

[0074] A sheet (1), of multilayer structure comprising an EVOH barrierlayer (9), a HDPE layer (8) and a layer (10) made of a HDPE filled with0.2% by weight of carbon black, was moulded by thermoforming so as toobtain a slimmed-down profile around a peripheral region (13), (14) witha width (l) of 10 mm until a thickness (e) of 0.8 mm was obtained in thecase of the layer of carbon-black-filled HDPE. This sheet (1) was weldedto the outer surface (12) of the tank by hot-plate welding, thecarbon-black-filled layer (10) being placed in contact with the wall ofthe tank (11). The slimmed-down peripheral region (13) (14) of the sheet(1) was then welded by a laser beam being scanned over the outer partsof the tank adjoining the hot-plate-welded region (12). The laseremployed was a FAP-type YAG diode laser with a wavelength of 809 nm anda power of 35 W, used in pulsed mode.

1. System for closing off an opening in a plastic tank, formed from aplastic sheet welded to the wall of the tank around the perimeter of theopening, the sheet being formed from a multilayer structure thatincludes a barrier layer, the outer layer of which structure, welded tothe tank, has a composition compatible with that of the plastic formingthe outer surface of the tank, characterized in that a peripheral regionof the sheet, in the weld region, has been slimmed down so as to adaptits geometry according to the following relationship: l/e≧5 in which lrepresents the width of the slimmed-down region and e the thickness ofthe layer in contact with the wall of the tank, both expressed in thesame length units.
 2. System according to the preceding claim,characterized in that the l/e ratio is at least
 8. 3. System accordingto claim 1 or 2, characterized in that the tank is formed from amultilayer structure.
 4. System according to the preceding claim,characterized in that the tank and the sheet are formed from the samemultilayer structure.
 5. System according to any one of the precedingclaims, furthermore characterized in that the sheet is fastened to atleast one accessory located on the inside of the tank.
 6. Method ofclosing off an opening in a plastic tank by means of a plastic sheet ofmultilayer structure, the outer layer of which, welded to the wall ofthe tank around the perimeter of the opening, has a compositioncompatible with that of the plastic forming the outer surface of thetank, characterized in that a peripheral region of the sheet is slimmeddown, before welding, so as to adapt its geometry according to thefollowing relationship: l/e≧5 in which l represents the width of theslimmed-down region and e its thickness, both expressed in the samelength units.
 7. Method according to the preceding claim, characterizedin that the slimmed-down part is welded by a laser radiation or infraredradiation welding technique.
 8. Method according to the preceding claim,characterized in that a composition opaque to the laser radiation or tothe infrared radiation is incorporated into the plastic layer of theslimmed-down region in contact with the outer surface of the tank. 9.Method according any one of claims 6 to 8, characterized in that theperipheral region of the sheet is slimmed down using a process chosenfrom thermoforming and extrusion-compression moulding.
 10. Use of themethod according to any one of claims 6 to 9 to close off an opening ina fuel tank intended for a motor vehicle.